Tau and Intracellular Transport in Neurons

Among the early changes in the brains of Alzheimer’s disease patients is the loss of synapses, which is accompanied by the abnormal phosphorylation of tau protein, its missorting into the somatodendritic compartment of neurons, and its incipient aggregation. The physiological function of tau is to stabilize axonal microtubules, which enables them to carry out their role as tracks for the transport of vesicles and organelles. By implication, perturbations in the functions of tau could be related to the loss of synapses and neuronal degeneration. Cell and transgenic animal models of tauopathy reveal that tau can indeed cause an impairment of transport in neurons. As a result, cell processes of neurons become starved, leading first to the decay of synapses and then to the loss of axons and dendrites. 1 Tau Protein: Properties and Functions The loss of synapses observed during incipient Alzheimer’s disease (AD) corresponds to the beginning loss of memory during the mild cognitive impairment phase (Coleman and Yao 2003). The synapse decay precedes the abnormal protein aggregation of the Aβ peptide in senile plaques or of tau protein in neurofibrillary tangles (Walsh and Selkoe 2004). It has been suspected that the highly elongated structure of neurons is one reason for their vulnerability. Most synapses are distant from the cell body, the major site of protein synthesis, and therefore rely on an efficient transport system. In cells, the traffic system consists of microtubules and microfilaments along which cargoes can be moved (Hollenbeck and Saxton 2005; Baas et al. 2006). This transport is achieved by means of motor proteins that can be subdivided into three classes: myosins (for the microfilament tracks), kinesins and dyneins (for microtubule tracks; Hirokawa and Takemura 2005). The directionality of movement E.-M. Mandelkow Max-Planck-Unit for Structural Molecular Biology, 22607 Hamburg, Germany P. St. George-Hyslop et al. (eds.) Intracellular Traffic and Neurodegenerative Disorders, Research and Perspectives in Alzheimer’s Disease, c © Springer-Verlag Berlin Heidelberg 2009 57 U nc or re ct ed P ro of 58 E.-M. Mandelkow et al. is determined by the polarity of the tracks and the directionality of the motors. The “plus” ends of microtubules point to the cell periphery, so that plus end-directed motors (kinesin) carry out anterograde transport and minus end-directed motors (dynein) achieve retrograde movements towards the cell body. The “ties” for the tracks are represented by microtubule-associated proteins (MAPs; Cassimeris and Spittle 2001). In neurons, the most important MAPs are MAP2 (mostly dendritic), tau and MAP1b (mostly axonal). The interaction of MAPs with microtubules is controlled by phosphorylation and involves several protein kinases and phosphatases (Mandelkow et al. 2007). Microtubules can assemble from their subunits (α-βtubulin heterodimers) under the regulation by GTP. Additional control is achieved by MAPs, such as tau, whose detachment can induce microtubule breakdown, and by the microtubule-disassembling proteins, katanin, spastin, or kinesin-13 (Howard and Hyman 2007). Tau has received attention in the field of several neurodegenerative disorders (“tauopathies”) because of its anomalous behavior (Ballatore et al. 2007, Schneider and Mandelkow 2008), which is most conspicuously seen as aggregation into neurofibrillary tangles, consisting of paired helical filaments (PHFs) and straight filaments (Crowther and Goedert 2000; Mandelkow et al. 2007). Tau also becomes highly phosphorylated, missorted into the somatodendritic compartment, partly cleaved by proteases, and otherwise modified (Watanabe et al. 2004; Binder et al. 2005). The H1 haplotype of tau shows a genetic association with certain tauopathies, e.g., progressive supranuclear palsy, corticobasal degeneration, AD and Parkinson disease, which may be caused by a perturbation of tau isoform homeostasis resulting in a relative increase of 4-repeat tau isoforms (inclusion of exon 10) and decrease of N-terminal inserts (especially lack of exon 3; Myers et al. 2007; Caffrey et al. 2007). Biochemically, AD-tau is found to be detached from microtubules and no longer stabilizes microtubules. The consequences are the destabilization of transport tracks and the aggregation of tau in the cytosol, both of which can disrupt intracellular traffic. AD-tau aggregates show a well-defined pattern of spreading in the brain, from the transentorhinal region to the hippocampus and later throughout the cortex. This pattern corresponds to the progression of clinical symptoms from mild cognitive impairment to severe dementia (Braak stages 1–6; Braak and Braak 1991). The gene of tau (MAPT) is located on chromosome 17; the protein occurs in the CNS as six main isoforms arising from alternative splicing (352–441 residues; Andreadis 2005). The repeat domain (3 or 4 pseudo-repeats of ∼31 residues, depending on splice isoforms) and the domains flanking the repeats are responsible for microtubule binding. The repeat domain also forms the core of Alzheimer PHFs (Wille et al. 1992; Novak et al. 1993). The overall character of tau is basic and hydrophilic, due to the many lysine or arginine and polar residues, which makes tau highly soluble, up to the point that tau is heat and acid stable without losing its biological function (Lee et al. 1988). A further consequence is that tau is not compactly folded as most proteins but rather is a natively unfolded protein (Schweers et al. 1994). Several mutations in the tau gene can cause different forms of neurodegeneration (FTDP-17; Ballatore et al. 2007), presumably due to a change in protein U nc or re ct ed P ro of Tau and Intracellular Transport in Neurons 59 function or an altered distribution of isoforms caused by modifications in the pattern of alternative splicing (D’Souza and Schellenberg 2005). Tau from AD brains is extensively phosphorylated, ∼4-fold higher than in normal brain and at numerous sites (Khatoon et al. 1992; Morishima-Kawashima et al. 1995). The consequences are heterogeneous. Phosphorylation at certain sites can affect microtubule binding and/or PHF aggregation; other sites appear to be functionally neutral (Schneider et al. 1999). Phosphorylation at the KXGS motifs in the repeat domain by the kinase MARK strongly disrupts tau-microtubule binding and leads to dynamic microtubules (Drewes et al. 1997). The interplay between tau and MARK becomes particularly noticeable in the case of neurite outgrowth, where activation of MARK has a similar effect as nerve growth factor signalling (Biernat et al. 2002). The most unusual property of tau in AD is its aggregation, which is counterintuitive because of the high solubility of tau. The aggregation is based on certain hexapeptide motifs in the sequence that have an increased propensity for β-sheet interactions (275VQIINK280 and 306VQIVYK311; von Bergen et al. 2000). Therefore, the aggregation of tau is based on an “amyloid” principle, although the major part of the protein remains disordered, even when it is assembled into PHFs. This finding is borne out by recent structural results. X-Ray crystallography reveals that amyloidogenic peptides derived from different proteins form hairpin-like “amyloid spines” that assemble into cross-β-sheets, stabilized by internal hydrophobic interactions and hydrogen bonds and paired by hydrophilic interactions (Sawaya et al. 2007). Nuclear magnetic resonance studies reveal that the amyloidogenic subdomains have an enhanced tendency for extended conformation with β-propensity even in solution, which is stabilized in hairpin-like conformations during fiber assembly (Mukrasch et al. 2005; Andronesi et al. 2008). 2 Tau and Transport Inhibition in Neurons The traffic systems in neurons can be regulated at different levels, for example at the level of tracks (microtubules, tau), motors (kinesin, dynein), or cargo adaptors (kinesin or dynein light chains or associated proteins), or by posttranslational modifications (phosphorylation; Stokin and Goldstein 2006). In this context, proteins closely related to AD include tau and protein kinases that can regulate tracks, motors, or adaptors. In cells, one observes that elevation of tau causes a stabilization of microtubules as well as a general inhibition of intracellular traffic, particularly in the anterograde direction (Stamer et al. 2002; Fig. 1), which can be explained by the fact that tau inhibits both forward motors (kinesin) and reverse motors (dynein), but the inhibition of kinesin is more pronounced, resulting in a net retrograde bias in the transport of vesicles and organelles (Seitz et al. 2002; Dixit et al. 2008). The observations are consistent with the view that the attachment of motors is obstructed by tau bound to the microtubule tracks. In addition, tau may interact directly with kinesin or dynein motors (Magnani et al. 2007; Cuchillo-Ibanez et al. 2008). The U nc or re ct ed P ro of 60 E.-M. Mandelkow et al.